U.S. patent application number 14/391950 was filed with the patent office on 2015-03-05 for method of measurement on a machine tool and corresponding machine tool apparatus.
This patent application is currently assigned to RENISHAW PLC. The applicant listed for this patent is RENISHAW PLC. Invention is credited to Paul Moore, John Ould, Michael Wooldridge.
Application Number | 20150066196 14/391950 |
Document ID | / |
Family ID | 49382988 |
Filed Date | 2015-03-05 |
United States Patent
Application |
20150066196 |
Kind Code |
A1 |
Wooldridge; Michael ; et
al. |
March 5, 2015 |
METHOD OF MEASUREMENT ON A MACHINE TOOL AND CORRESPONDING MACHINE
TOOL APPARATUS
Abstract
A method of scanning an object using an analogue probe mounted
on a machine tool, so as to collect scanned measurement data along
a nominal measurement line on the surface of the object, the
analogue probe having a preferred measurement range. The method
includes controlling the analogue probe and/or object to perform a
scanning operation in accordance with a course of relative motion,
the course of relative motion being configured such that, based on
assumed properties of the surface of the object, the analogue probe
will be caused to obtain data within its preferred measuring range,
as well as cause the analogue probe to go outside its preferred
measuring range, along the nominal measurement line on the surface
of the object.
Inventors: |
Wooldridge; Michael;
(Stroud, GB) ; Moore; Paul; (Stroud, GB) ;
Ould; John; (Backwell Farleigh, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RENISHAW PLC |
Wotton-under-Edge, Gloucestershire |
|
GB |
|
|
Assignee: |
RENISHAW PLC
Wotton-under-Edge, Gloucestershire
GB
|
Family ID: |
49382988 |
Appl. No.: |
14/391950 |
Filed: |
April 16, 2013 |
PCT Filed: |
April 16, 2013 |
PCT NO: |
PCT/GB2013/050968 |
371 Date: |
October 10, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61720293 |
Oct 30, 2012 |
|
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Current U.S.
Class: |
700/195 |
Current CPC
Class: |
G05B 19/401 20130101;
G05B 2219/50063 20130101; G01B 21/045 20130101 |
Class at
Publication: |
700/195 |
International
Class: |
G05B 19/401 20060101
G05B019/401 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 18, 2012 |
EP |
12250096.0 |
Claims
1. A method of scanning an object using a contact analogue probe
comprising a deflectable stylus for contacting the object mounted
on a machine tool, so as to collect scanned measurement data along
a nominal measurement line on the surface of the object, the
analogue probe having a preferred measurement range, the method
comprising: controlling the contact analogue probe and/or object to
perform a scanning operation in accordance with a course of
relative motion, the course of relative motion being configured
such that, based on assumed properties of the surface of the
object, the analogue probe will be caused to obtain data within its
preferred measuring range, as well as cause the analogue probe to
go outside its preferred measuring range, along the nominal
measurement line on the surface of the object.
2. A method as claimed in claim 1, comprising collecting and
outputting scanned measurement data obtained within the contact
analogue probe's preferred measuring range as the measurement of
the object.
3. A method as claimed in claim 2, in which said collecting and
outputting comprises filtering the data obtained from the contact
analogue probe so as to obtain, and provide as the measurement of
the object, select object measurement data obtained from within the
contact analogue probe's preferred measurement range.
4. A method as claimed in claim 1, in which the contact analogue
probe proceeds in a manner that, based on assumed properties of the
surface of the object, causes the position of the contact analogue
probe's preferred measurement range to repeatedly rise and fall
relative to the surface of the object as it moves along the nominal
measurement line.
5. A method as claimed in claim 1, in which the contact analogue
probe is maintained in a position sensing relationship with the
surface of the object as it is moved so as to collect data along
the nominal measurement line.
6. A method as claimed in claim 1, in which the course of relative
motion is configured such that during the scanning operation the
contact analogue probe's preferred measuring range traverses across
the object along the nominal measurement line a plurality of
times.
7. A method as claimed in claim 6, in which for different traverses
the contact analogue probe obtains measurement data within its
preferred measurement range for different parts of the object along
the nominal measurement line.
8. A method as claimed in claim 6, in which the form of the route
the preferred measuring range takes relative to the surface is
substantially the same for successive traverses, but in which
height of the route from the surface at at least one point along
the nominal measurement line is different for different
traverses.
9. A method as claimed in claim 6, in which surface measurement
data obtained within the preferred measuring range from different
traverses is collated so as to form a measurement data set
representing the surface of the object along the nominal
measurement line.
10. A method as claimed in claim 1, comprising generating and
executing, as part of a second scanning operation, a new course of
relative movement of the analogue probe and object.
11. A method as claimed in claim 10, in which the new course of
relative movement comprises the contact analogue probe traversing
substantially the same line of measurement across the surface of
the object, but in which the relative movement is controlled such
the contact analogue probe obtains measurements within its
preferred measurement range for a greater proportion of the course
of movement than for that of the scanning operation.
12. A method as claimed in claim 1, in which the contact analogue
probe's preferred measurement range is a preferred range of
deflection of the contact analogue probe.
13. A computer program comprising instructions which when executed
by a machine tool apparatus causes the machine tool apparatus to
perform the method of claim 1.
14. A computer readable medium comprising instructions which when
executed by a machine tool apparatus causes the machine tool
apparatus to perform the method of claim 1.
15. A machine tool apparatus comprising a machine tool, a contact
analogue probe having a deflectable stylus mounted on the machine
tool, and a controller configured to control the relative movement
of the contact analogue probe and an object to be measured so as to
so as to collect scanned measurement data along a nominal
measurement line on the surface of the object, and in particular so
as to control the contact analogue probe and/or object in
accordance with a course of relative motion such that the position
of the preferred measuring range relative to the surface of the
object is controlled in a manner that, based on assumed properties
of the surface of the object, will cause the contact analogue probe
to obtain data within its preferred measuring range, as well as to
exceed its preferred measuring range, along the nominal measurement
line on the surface of the object.
16. A method of scanning an object using an analogue probe mounted
on a machine tool, the analogue probe having a preferred
measurement range, the method comprising performing a scanning
measurement operation which comprises moving the object and
analogue probe relative to each other so that the analogue probe
obtains scanned measurement data along a nominal measurement line
on the surface of the object, in which some of the data obtained
during the scanning measurement operation along the nominal
measurement line is within the analogue probe's preferred
measurement range and some is outside the probe's preferred
measurement range.
Description
[0001] This invention relates to a method of measuring an artefact,
and in particular to a method of scanning an artefact using an
analogue measurement tool mounted on a machine tool.
[0002] It is known to mount a measurement probe in a machine tool
spindle, for movement with respect to a workpiece, in order to
measure the workpiece. In practice, the probe has typically been a
touch trigger probe, e.g. as described in U.S. Pat. No. 4,153,998
(McMurtry), which produces a trigger signal when a stylus of the
probe contacts the workpiece surface. This trigger signal is taken
to a so-called "skip" input of the machine tool's numeric
controller (NC). In response, relative movement of the object and
workpiece are stopped and the controller takes an instantaneous
reading of the machine's position (i.e. the position of the spindle
and the probe relative to the machine). This is taken from
measurement devices of the machine such as encoders which provide
position feedback information in a servo control loop for the
machine's movement. A disadvantage of using such a system is that
the measurement process is relatively slow resulting in long
measurement times if a large number of measurement points are
required.
[0003] Analogue measurement probes (also commonly known as scanning
probes) are also known. Contact analogue probes typically comprise
a stylus for contacting the workpiece surface, and transducers
within the probe which measure the deflection of the stylus
relative to the probe body. An example is shown in U.S. Pat. No.
4,084,323 (McMurtry). In use, the analogue probe is moved relative
to the surface of the workpiece, so that the stylus scans the
surface and continuous readings are taken of the outputs of the
probe transducers. Combining the probe deflection output and the
machine position output allows co-ordinate data to be obtained
thereby allowing the position of the workpiece surface to be found
at a very large number of points throughout the scan. Analogue
probes thus allow more detailed measurements of the form of the
workpiece surface to be acquired than is practically possible using
a touch trigger probe.
[0004] As will be understood (and explained in more detail below in
connection with FIG. 2), an analogue probe has a limited
measurement range. Furthermore, the analogue probe might have a
preferred measurement range. The analogue probe might be able to
obtain data outside its preferred measurement range, but the data
obtained outside this range could be less preferred, for instance
because it could be considered to be less accurate than the data
obtained within the preferred measuring range. The boundaries of
the preferred measurement range can vary depending on many
different factors, including the type of probe, the calibration
routine used, and even for instance the object being measured. In
many circumstances it can be preferred to ensure that the analogue
probe is kept within its preferred measurement range as it scans
along the surface of the workpiece. The preferred measurement range
of an contact analogue probe can be for example +/-0.8 mm in any
given dimension or smaller, for example in some circumstances as
small as +/-0.3 mm in any given dimension. (These values could be
measured from the stylus' rest position). Furthermore, the actual
preferred measurement range could be even smaller than the figures
given above because it might be that a minimum amount of deflection
could be needed to enter the preferred measuring range.
Accordingly, although the preferred measuring range might be +/-0.5
mm from the rest position, at least the first +/-0.05 mm of
deflection or for example the first +/-0.1 mm of deflection might
not be within the preferred measuring range (again, this is
explained in more detail below in connection with FIG. 2).
Accordingly, as will be understood, real-time management of the
probe/workpiece positional relationship is required to avoid
situations in which the analogue probe falls outside its preferred
measurement range.
[0005] This is why analogue probes have typically only been used
with dedicated co-ordinate measuring machines (CMMs) even though
analogue probes have been known per se for many years. This is
because CMMs have dedicated real-time control loops to enable such
management of probe deflection to occur. In particular, in CMMs a
controller is provided into which a program is loaded which defines
a predetermined course of motion for the measurement probe to move
along relative to a workpiece. The controller generates motor
control signals from the program which are used to activate motors
to induce movement of the measurement probe. The controller also
receives real-time position data from the machine's encoders and
also deflection data (in the case of a contact probe) from the
analogue probe. In order to accommodate for variations in the
material condition of the workpiece a dedicated control loop
arrangement exists. This comprises a feedback module into which the
above mentioned motor control signals and deflection data are fed.
The feedback modules uses logic to continuously update (based on
the deflection data) an offset control vector which in turn is used
to adjust the above mentioned motor control signal generated from
the program before it is sent to the CMM's motors so as to try to
maintain probe deflection within the preferred measuring range as
the analogue probe scans the workpiece. This all happens within a
closed loop control loop with a response time of less than 1 to 2
ms. This is for example described in WO2006/115923.
[0006] Such tight control over probe positioning plus the ability
to process real-time stylus deflection data allows such dedicated
CMMs to scan complex articles that deviate from their expected
shape and even to scan articles of unknown shape.
[0007] To date, analogue probes have not been widely used for
machine tool scanning applications. This is due to the inherent
nature of many commercially available machine tools which do not
facilitate the real-time control of the analogue probe that CMMs
provide. This is because machine tools are primarily developed to
machine workpieces and the use of measurement probes on them to
measure workpieces is essentially an after-thought. Machine tools
are therefore typically not configured for real-time control using
data from an analogue measurement probe. Indeed, it is often the
case that the machine tool has no in-built provision for the direct
receipt of deflection data from the measurement probe. Rather, the
probe has to communicate (e.g. wirelessly) with an interface which
receives the probe deflection data and passes the data to a
separate system which subsequently combines the deflection data
with machine position data so as to subsequently form complete
object measurement data, for instance as described in
WO2005/065884.
[0008] This makes it difficult to use an analogue probe on a
machine tool to obtain scanned measurement data about known
objects, because any variation from the expected shape of the
object can cause the probe to over deflect and hence cause the
measurement process to fail (whereas on a CMM the probe's course of
motion could be updated quickly enough to ensure that the probe
doesn't over deflect). This also makes it difficult to use an
analogue probe on a machine tool to obtain scanned measurement data
about unknown objects because this inherently requires the probe's
course of motion to be updated quickly enough so as to avoid over
deflection.
[0009] Techniques for overcoming the problems of using an analogue
scanning probe on a machine tool have been developed. For instance,
drip feed techniques are known in which the program instructions
are loaded into the machine tool's controller in a drip fed manner.
In particular, each instruction causes the probe to move by a tiny
distance (i.e. less than the probe's preferred deflection range),
and the probe's output is analysed to determine the extent of
deflection, which in turn is used to generate the next instruction
to be fed into the controller. However, such a technique is still
much more limited than the scanning techniques that can be
performed using an analogue scanning probe on a CMM. In particular,
such a method is very slow and inefficient.
[0010] WO2008/074989 describes a process for measuring a known
object which involves repeating a measurement operation according
to an adjusted path if a first measurement operation resulted in
over or under deflection.
[0011] The problem can also be further compounded when using
analogue probes on machine tools because due to their construction
(which enables them to be able to used within the harsher
environments that machine tools provide and the greater
accelerations and forces they are exposed to such as when they are
auto-changed into/out of a machine tool's spindle) they often have
a much smaller measurement range than those analogue probes which
are for use with CMMs. This can therefore give even less room for
error compared to analogue probes used on CMMs. For example, a
machine tool analogue probe could have a measurement range of
+/-0.8 mm in any given dimension or smaller (measured from the
stylus' rest position), for example in some circumstances +/-0.5 mm
in any given dimension or smaller, and for example in some
circumstances no bigger than +/-0.3 mm in any given dimension. This
can therefore give even less room for error compared to analogue
probes used on CMMs. As mentioned above, a minimum deflection might
also be required in order to enter the preferred measuring
range.
[0012] As a specific example, the measurement range could be
defined by a maximum deflection 0.725 mm and a minimum deflection
of 0.125 mm (measured from the stylus' rest position). Accordingly,
in this case, this can mean that the surface can be +/-0.3 mm from
nominal whilst maintaining an accurate measurement. However, this
figure can be smaller, and for instance it is known to for surface
uncertainties to be as small as +/-0.1 mm, which corresponds to a
maximum probe deflection of around +/-0.325 mm and a minimum probe
deflection of +/-0.125 mm.
[0013] According to a first aspect of the invention there is
provided, a method of scanning an object using an analogue probe
mounted on a machine tool, so as to collect scanned measurement
data along a nominal measurement line on the surface of the object,
the analogue probe having a preferred measurement range, the method
comprising: controlling the analogue probe and/or object to perform
a scanning operation in accordance with a course of relative
motion, the course of relative motion being configured such that
the position of the preferred measuring range relative to the
surface of the object is controlled in a manner that, based on
assumed properties of the surface of the object, will cause the
analogue probe to obtain data within its preferred measuring range,
as well as cause the analogue probe to be outside its preferred
measuring range, along the nominal measurement line on the surface
of the object.
[0014] Accordingly, rather than trying to always keep the analogue
probe within its preferred measurement range, the present invention
therefore works on the basis that it expects the analogue probe to
move within and outside its preferred measurement range along the
nominal measurement line on the surface of the object. This could
be such that it expects the analogue probe to obtain measurements
both within and outside its preferred measuring range along the
nominal measurement line on the surface of the object. Indeed, the
method can be configured such that the position of the analogue
probe's preferred measurement range with respect to the surface of
the object is controlled such that the analogue probe is caused to
deliberately obtain measurements both within and outside its
preferred measurement range, along the nominal measurement line on
the surface of the object. This can improve the efficiency by which
object measurement data is obtained using an analogue probe on a
machine tool.
[0015] It could be that the course of relative motion is configured
such that the position of the preferred measuring range relative to
the surface of the object is controlled in a manner that, based on
assumed properties of the surface of the object, will cause the
analogue probe to obtain data within its preferred measuring range,
as well as cause the analogue probe to exceed its preferred
measuring range, along the nominal measurement line on the surface
of the object. It could be that the course of relative motion is
configured such that the position of the preferred measuring range
relative to the surface of the object is controlled in a manner
that, based on assumed properties of the surface of the object,
will cause the analogue probe to obtain data within its preferred
measuring range, as well as cause the analogue probe to exceed and
or fall-short of its preferred measuring range, along the nominal
measurement line on the surface of the object.
[0016] The method could be configured such that the course of
relative motion is configured such that during the scanning
operation, based on assumed properties of the surface of the
object, the position of the preferred measuring range relative to
the surface of the object in a direction normal to the surface of
the object (e.g. the height) varies along the nominal measurement
line.
[0017] The method can further comprise filtering the data obtained
from the analogue probe so as to obtain select scanned measurement
data. The method can comprise filtering the data obtained from the
analogue probe so as to obtain data relating to scanned measurement
data obtained predominantly from either within or from outside the
analogue probe's preferred measurement range. The method can
comprise filtering the data obtained from the analogue probe so as
to obtain select scanned measurement data relating predominantly to
scanned measurement data obtained from within the analogue probe's
preferred measurement range. The method can comprise filtering the
data obtained from the analogue probe so as to obtain select
scanned measurement data relating substantially to only scanned
measurement data obtained from within the analogue probe's
preferred measurement range.
[0018] The method can comprise collating said filtered data into a
further data set. Accordingly, for instance, the further data set
could comprise scanned measurement data relating to the surface of
the object that was obtained within the analogue probe's preferred
measurement range. The further data set could be output as
measurement data of the object.
[0019] Accordingly, the method can comprise collecting and
outputting scanned measurement data obtained within the analogue
probe's preferred measuring range as the measurement of the object.
In line with the above, such collecting and outputting can comprise
filtering the data obtained from the analogue probe so as to
obtain, and provide as the measurement of the object, select object
measurement data obtained from within the analogue probe's
preferred measurement range.
[0020] The preferred measuring range can be less than the total
measuring range of the analogue probe. In the case of a contact
probe, the preferred measuring range can be less than the total
deflection range of the analogue probe. Accordingly, the preferred
measuring range could be a subset of the analogue probe's entire
measurement range. As mentioned above, the exact boundaries of the
preferred measurement range can vary from probe to probe and even
from measurement operation to measurement operation for any given
probe. It could be the range for which the analogue probe has been
calibrated for any given measurement operation, e.g. to give a
desired level of accuracy.
[0021] The method can comprise generating and executing (e.g. as
part of a second scanning operation) a new course of relative
movement of the analogue probe and object based on the measurement
data obtained during the previous scanning operation. The new
course of relative movement can comprise the analogue probe
traversing substantially the same line of measurement across the
surface of the object. However, in this case the relative movement
can be controlled such that the relative position of the analogue
probe and object is such that the analogue probe obtains
measurements within its preferred measurement range for a greater
proportion of the measurement path than for the previous
measurement of the object. In particular, the new path of relative
movement for the analogue probe and object to follow can be
configured such that the analogue probe obtains measurement data
within its preferred measurement range along substantially the
entire length of the same nominal line.
[0022] The object and analogue probe could be configured to move
relative to each other along a predetermined path of relative
motion so that the analogue probe obtains scanned measurement data
along the nominal measurement line on the surface of the
object.
[0023] The predetermined path of relative motion can be configured
such that the analogue probe proceeds in a manner that, based on
assumed properties of the surface of the object, causes the
position of the analogue probe's preferred measurement range to
repeatedly rise and fall relative to the surface of the object as
it moves along the nominal measurement line. Accordingly, this
could be so as to cause, based on assumed properties of the surface
of the object, the analogue probe to oscillate between obtaining
data within and outside its preferred measuring range (e.g. under
and within, or within and beyond, or under, within and beyond
preferred measuring range) along the nominal measurement line. For
instance, the predetermined path of relative motion can be
configured such that the analogue probe moves in an undulating,
sinusoidal or wavy manner as it moves along the nominal measurement
line.
[0024] The predetermined path of relative motion can be configured
such that, based on assumed properties of the surface of the
object, the analogue probe is maintained in a position sensing
relationship with the surface of the object as it is moved along
the nominal measurement line. This could particularly be the case
with the above described embodiment in which the predetermined path
of relative motion is configured such that the analogue probe
proceeds in a manner that, based on assumed properties of the
surface of the object, causes the position of the analogue probe's
preferred measurement range to repeatedly rise and fall relative to
the surface of the object as it moves along the nominal measurement
line.
[0025] The course of relative motion could be configured such that
during the scanning operation the analogue probe's preferred
measuring range traverses across the object along the nominal
measurement line a plurality of times. The positional relationship
of the analogue probe and object can be different for different
traverses. The analogue probe can obtain measurement data within
different regions of its entire measuring range for different
traverses. The form of the route the analogue probe and object take
relative to each other can be substantially the same for each
traverse such that the object is measured along the nominal
measurement line on the surface of the object a plurality of times.
However, the position of the analogue probe and object could be
offset relative to each other for different traverses.
[0026] Accordingly, the course of motion could be configured such
that for different traverses the analogue probe obtains measurement
data within its preferred measurement range for different parts of
the object, along the same nominal measurement line on the surface
of the object. The form of the route that the preferred measuring
range takes relative to the surface can be substantially the same
for successive traverses. Accordingly, the height of the route from
the surface at at least one point along the nominal measurement
line (and preferably along the entire length of the nominal
measurement line) can be different for different traverses. In
particular, the traverses can be offset from each other, such that
for different traverses the analogue probe obtains measurement data
within its preferred measurement range for different parts of the
object, along the same nominal measurement line on the surface of
the object. In other words, the course of motion could be
configured such that the preferred measuring range traverses across
the object along the nominal measurement line, at least twice, each
traverse being substantially parallel to each other but at
different nominal heights to the surface of the object. The nominal
height could increase over successive traverses. Preferably, the
nominal height decreases over successive traverses.
[0027] Accordingly, the course of motion could be configured such
that at least a first and second traverses are performed, and in
which during the second traverse the analogue probe obtains
measurement data within its preferred measurement range for at
least a part of the object for which data was obtained outside of
the probe's preferred measurement range during the first
traverse.
[0028] As mentioned above, the position of the analogue probe's
preferred measurement range above the surface of the object can be
different for different traverses. The position of the analogue
probe's preferred measurement range relative to the surface of the
object could rise over successive traverses. Preferably, the
position of the analogue probe's preferred measurement range
relative to the surface of the object falls over successive
traverses. The position could be measured between a reference point
with respect to the preferred measuring range (e.g. a point within
the preferred measuring range, such as the mid-point of the
preferred measuring range) and the surface of the object (e.g. the
nominal surface of the object). Accordingly, for instance,
preferably the line along which the centre of the preferred
measuring range follows for each pass could, on average,
progressively fall (e.g. get closer to/penetrate deeper into) with
respect to the surface of the object over successive traverses.
This could happen in a step-by-step manner, e.g. at the end of each
traverse.
[0029] The course of relative motion can be configured such that
the difference between previous and subsequent traverses is
sufficiently small such that if along the previous traverse no
surface measurement data was obtained, the subsequent traverse will
not cause the analogue probe to obtain object surface measurement
data that exceeds its entire measurement range, and for example
will not cause the analogue probe to obtain data beyond its
preferred measuring range. Optionally, traverses are offset from
each other in steps that are no bigger than, and for instance are
smaller than, the entire measurement range of the probe. For
example, traverses can be offset from each other in steps that are
no bigger than, and for instance are smaller than, the preferred
measuring range of the probe.
[0030] Surface measurement data obtained within the preferred
measuring range from different traverses can be collated so as to
form a measurement data set which represents the surface of the
object along the nominal measurement line. As mentioned above, the
course of motion can be configured such that for different
traverses the analogue probe obtains measurement data within its
preferred measurement range for different parts of the object,
along the same nominal measurement line on the surface of the
object. Preferably, the course of motion is configured such that
portions of the surface for which measurement data is obtained
within the preferred measurement range overlap between successive
passes. In this case, the measurement data set could represent a
continuous length of the surface along the nominal measurement
line, and preferably represent the surface along the entire length
of the nominal measurement line. However, it might be that the
portions do not overlap which could therefore mean that might be
gaps in the measurement data set.
[0031] The nominal surface shape of the object might not be known.
The nominal surface shape of the object could be known. In this
case the shape of the measurement path across the object can be
configured to be substantially parallel to nominal surface shape.
That is the path across the object the preferred measurement range
is configured to take can be configured to be substantially
parallel to the nominal surface shape.
[0032] The analogue probe could be a non-contact analogue probe,
for instance an optical, capacitance or inductance probe. In this
case, the preferred measurement range could be a distance or
separation range between a part of the analogue probe (e.g. the
workpiece sensing part) and the workpiece surface. Accordingly, the
preferred measurement range could comprise upper and lower
boundaries or thresholds relating to maximum and minimum
probe-object separations. The analogue probe can be a contact
analogue probe. For instance, the analogue probe could be a contact
analogue probe with a deflectable stylus for contacting the object.
In this case, the preferred measurement range can be a preferred
stylus deflection range. Accordingly, the preferred measurement
range could comprise upper and lower boundaries or thresholds
relating to maximum and minimum stylus deflections.
[0033] The object could be an object that was (and/or is to be)
machined on the machine on which the analogue probe is mounted.
Accordingly, the method could comprise, the same machine tool
machining the object, for example prior to the above described
measuring steps. Optionally machining could take place after the
above described measuring steps. Such post-measurement machining
could take place on the same machine tool on which the measurement
occurred. Such post-measurement machining could be based on
measurement data obtained during the above described measurement
steps. The machine tool could be a cutting machine, such as a metal
cutting machine.
[0034] The analogue probe could be a sealed analogue probe. That is
the analogue probe could be sealed so as to protect internal sensor
componentry from external contaminants. For instance, the probe
could comprise a probe body which houses a sensor for either
directly or indirectly measuring the surface of an object, in which
the sensor is sealed from external contaminant. For instance, in
the case of a contact probe, the probe could comprise a probe body,
a stylus member and a sensor for measuring displacement of the
stylus member relative to the housing, in which at least a first
compliant sealing member is provided which extends between the
probe body and relatively moveable stylus member, such that the
sensor is contained within a sealed chamber and thereby sealed from
external contaminants.
[0035] The object can be a blade. For instance, the blade could be
a blade of a turbine engine.
[0036] Accordingly, this application describes a method of scanning
an object using an analogue probe mounted on a machine tool, the
analogue probe having a preferred measurement range, the method
comprising: performing a scanning measurement operation which
comprises moving the object and analogue probe relative to each
other so that the analogue probe obtains scanned measurement data
along a nominal measurement line on the surface of the object, in
which some of the data obtained during the scanning measurement
operation along the nominal measurement line is within the analogue
probe's preferred measurement range and some is outside the probe's
preferred measurement range.
[0037] According to a second aspect of the invention there is
provided a computer program comprising instructions which when
executed by a machine tool apparatus causes the machine tool
apparatus to perform the above described method.
[0038] According to a third aspect of the invention there is
provided a computer readable medium comprising instructions which
when executed by a machine tool apparatus causes the machine tool
apparatus to perform the above described method.
[0039] According to a fourth aspect of the invention there is
provided a machine tool apparatus comprising a machine tool, an
analogue probe mounted on the machine tool, and a controller
configured to control the relative movement of the analogue probe
and an object to be measured so as to so as to collect scanned
measurement data along a nominal measurement line on the surface of
the object, and in particular so as to control the analogue probe
and/or object in accordance with a course of relative motion such
that the position of the preferred measuring range relative to the
surface of the object is controlled in a manner that, based on
assumed properties of the surface of the object, will cause the
analogue probe to obtain data within its preferred measuring range,
as well as to exceed its preferred measuring range, along the
nominal measurement line on the surface of the object.
[0040] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying drawings in
which:
[0041] FIG. 1 is a schematic diagram showing the system
architecture for a machine tool;
[0042] FIGS. 2(a) to (c) are schematic diagrams illustrating the
measurement range of analogue measurement probes;
[0043] FIG. 3 is a system flow chart illustrating the flow of
control during a measurement operation according to an embodiment
of the invention;
[0044] FIG. 4 schematically illustrates the nominal path of a
stylus tip according to a first embodiment of the invention;
[0045] FIGS. 5(a) and 5(b) schematically illustrate side and
isometric views of a nominal path of a stylus tip according to a
second embodiment of the invention;
[0046] FIGS. 6(a) and 6(b) respectively illustrate nominal path of
a stylus tip according to a third and fourth embodiments of the
invention; and
[0047] FIG. 7 illustrates nominal paths of a stylus tip according
to a preliminary scan and a subsequent scan generated based on data
obtained during the preliminary scan, according to a further
embodiment of the invention.
[0048] Referring to FIG. 1, there is shown a machine tool apparatus
2 comprising a machine tool 4, a controller 6, a PC 8 and a
transmitter/receiver interface 10. The machine tool 4 comprises
motors (not shown) for moving a spindle 12 which holds an analogue
probe 14 relative to a workpiece 16 located on a table 15. The
location of the spindle 12 (and hence the analogue probe 14) is
accurately measured in a known manner using encoders or the like.
Such measurements provide spindle position data defined in the
machine co-ordinate system (x, y, z). A numerical controller (NC)
18 (which is part of the controller 6) controls x, y, z movement of
the spindle 12 within the work area of the machine tool and also
received data relating to the spindle position.
[0049] As will be understood, in alternative embodiments relative
movement in any or all of the x, y and z dimensions could be
provided by movement of the table 15 relative to the spindle.
Furthermore, relative rotational movement of the analogue probe 14
and workpiece 16 could be provided by a part of the spindle 12
(e.g. a rotating/articulated head mounted on the spindle) and/or a
part of table 15 (e.g. a rotary table). Furthermore, movement might
be restricted to fewer dimensions, e.g. only x, and/or y. Further
still, the embodiment described comprises a cartesian machine tool,
whereas will be understood this need not necessarily be the case
and could be instance be a non-cartesian machine tool. Further
still, many other different types of machine tools, including
lathes, and parallel-kinematic machines, and robot arms are known
and could be used with the invention.
[0050] In the embodiment described, the analogue probe 14 is a
contact analogue probe which comprises a probe body 20, a workpiece
contacting stylus 22 extending from the probe body 20, and has a
surface detection region in the form of a workpiece contacting tip
24 (which in this case in the form of a spherical stylus ball) at
the distal end of the stylus 22. The analogue probe 14 measures
deflection of the stylus 22 in a probe geometry system (a, b, c).
(However, as will be understood, this need not necessarily be the
case, and for instance the analogue probe could measure deflection
in only 1 or 2 dimensions, or even provide an output indicative of
the extent of deflection, without any indication of the direction
of deflection). The probe 14 also comprises a transmitter/receiver
(not shown) that wirelessly communicates with the
transmitter/receiver interface 10 (e.g. via a radio, optical or
other wireless transmission mechanism).
[0051] As mentioned above, analogue measurement probes have a
limited measurement range. For instance with regard to contact
analogue probes, they can have a physical maximum amount by which
they can be deflected in the x, y and z dimensions. Not only this,
but it can be that the probe is configured such that it works
optimally within a certain sub-range of the maximum physical range.
For instance, FIG. 2(a) illustrates the analogue probe of FIG. 1,
and the solid line represents the position of the stylus 22 at a
rest (e.g. undeflected) position. The outermost stylus positions
shown in dashed lines represent the maximum physical deflection of
the stylus in the x-dimension. However, it could be that the probe
is configured such that it is most accurate when the stylus is
deflected by an amount less than the maximum physical deflection.
It could also be that the probe is configured such that it is most
accurate when the stylus is deflected by a minimum lower threshold.
For instance, the analogue probe 14 could have a preferred
measurement range, the upper and lower boundaries of which are
shown by stylus positions shown in FIG. 2(a) as dotted lines.
Accordingly, as can be seen there is a dead space `d` (in the
x-dimension) in the middle close to the stylus' rest position which
is outside the preferred measuring range.
[0052] As will be understood, the same will also be the case with
deflection in the y-dimension. Furthermore, in the described
embodiment there is also a maximum physical deflection range in the
z-axis as well as a sub-range of z-axis deflections (a preferred
measurement range) within which the probe is configured to provide
the most accurate results.
[0053] The dotted line 28 shown in FIG. 2(b) schematically
illustrates the scope of the analogue probe's 14 preferred
measurement range taken in the x and z dimensions. As will be
understood, such a range actually extends in a three dimensions,
and hence is actually approximately the shape of a squashed
hemisphere with a small hole cut out in the middle.
[0054] The dotted lines of FIG. 2(c) also schematically illustrate
the preferred measurement range for a non-contact probe, such as an
inductance probe. The inner and outer dotted lines represent the
minimum and maximum probe/workpiece separation boundaries for
optimum measuring performance. As will be understood, the preferred
measuring range shown for the non-contact probe could be the entire
measuring range or only a subset of the entire measuring range for
the probe. As will be understood, the entire measuring range could
be considered to be what can be referred to as the non-contact
probe's surface detecting region.
[0055] As will be understood, the size of the preferred measuring
range will vary from probe to probe. For a contact analogue probe,
it could be for example not more than +/-0.8 mm in any given
dimension, for example not more than +/-0.725 mm in any given
dimension, for instance not more than +/-0.5 mm in any given
dimension, for example in some circumstances not more than +/-0.3
mm in any given dimension (taken from the stylus rest position). Of
course, there might also be a dead-zone immediately around the
stylus position through which the stylus has to be deflected beyond
before it enters the preferred measuring range, which could be for
example not less than +/-0.2 mm in any given dimension from the
stylus rest position, for instance not less than +/-0.1 mm in any
given dimension from the stylus rest position, e.g. not less than
+/-0.125 mm in any given dimension (again, measured from the stylus
rest position).
[0056] As described above, the present invention departs from the
traditional view that the probe must be maintained such that along
the nominal measurement line on the surface of the object the probe
always collects data within its preferred measurement range.
Rather, as is clear from the embodiments described below, the
invention enables measurements along the nominal measurement line
to be obtained both within and outside the probe's preferred
measurement range and then subsequently filtered as required.
[0057] FIG. 3 illustrates the general procedure 100 involved
according to one embodiment of the invention. The method starts at
step 102 at which point a model of the part to be measured is
loaded into the PC 8. As explained in more detail below, this step
need not necessarily be performed in embodiments in which the
workpiece to be measured is unknown. At step 104 a program defining
a course of motion for the analogue probe 14 to obtain scanned
measurement data of the workpiece 16 is generated. In the
embodiment described, the course of motion is configured such that
the analogue probe will obtain measurement data both within and
outside its preferred measurement range along a nominal measurement
line on the surface of the object. As will be understood, in
embodiments in which the workpiece 16 can be moved as well as, or
instead of the analogue probe 14 (e.g. by virtue of a movable table
15), then the program can also define a course of motion of the
workpiece 16. In other words, step 104 comprises planning the
relative course of motion between the analogue probe 14 and the
workpiece 16 so that the analogue probe 14 can collect scanned
measurement data regarding the workpiece 16. At step 106 the
program is loaded into the NC 18 via the API 26. Step 108 involves
performing the measurement operation and recording measurement
data. In particular, performing the measurement operation comprises
the NC 18 interpreting the program's instructions and generating
motor control signals which are used to instruct the machine tool's
4 motors (not shown) so as to move the analogue probe 14 in
accordance with the predetermined course of motion. Recording
measurement data comprises a number of procedures. In particular,
spindle position data (x, y, z) (which as mentioned above is
provided by encoders on the machine tool 4) is passed to the PC 8
via the NC 18. Furthermore, probe deflection data (a, b, c) (which
as mentioned above is obtained by the analogue probe) is also
passed to the PC 8 via the probe transmitter/receiver interface 10.
The PC 8 combines the spindle position data (x, y, z) and the probe
deflection data (a, b, c) to provide a set of measurements that
define the position of a surface within the machine co-ordinate
geometry.
[0058] Step 110 comprises the PC 8 filtering the recorded
measurement data. In the particular embodiment described, this
comprises the PC 8 filtering the recorded measurement data for
measurement data that was obtained within the analogue probe's
preferred measurement range. As will be understood, the data could
be filtered in other ways, for instance, for measurement data that
was obtained outside the analogue probe's preferred measurement
range. As will be clear from the different embodiments described
above, how the filtering is performed, and the end result obtained
varies from embodiment to embodiment.
[0059] For instance (and as explained in more detail below), FIG. 4
illustrates a technique according to the invention for measuring a
known part in which the stylus tip is moved across the workpiece 16
in an undulating manner so as to collect data within and outside
its preferred measurement range. FIG. 5 illustrates a technique for
measuring an unknown part by traversing back and forth across the
same nominal measurement line on the surface of the part at
different nominal stylus tip positions, and FIG. 6 illustrates a
similar technique but which is used for measuring a known part.
[0060] Referring first to FIG. 4, this illustrates that the stylus
tip 24 could be configured such that its gross movement
(illustrated by feint dotted line 30) is generally parallel to
surface 17 of the workpiece 16. However, as illustrated by the dark
dotted line 32, the relative course of motion of the analogue probe
and workpiece 16 is configured such that the nominal stylus tip
centre point 23 is caused to undulate toward and away from the
surface as it travels along a nominal measurement line 19 on the
surface 17 of the object 16. The dotted circles 24A, 24B, 24C
represent the nominal position of the stylus tip at three different
points along the nominal measurement line on the surface of the
object 16. As will be understood, these stylus tip positions 24A,
24B, 24C are nominal in that this is where the position the tip
would be in at those points along the nominal measurement line if
the object was not there. The nominal measurement line 19 (which is
more easily seen in FIG. 5(b) which illustrates a different
embodiment of the invention) is the line on the surface 17 of the
object on which measurement data is to be gathered. The line is
nominal in that this is the expected line of measurement on the
object. As will be understood, the actual line of measurement may
be different if the location and/or material condition of the
object 16 is different to what is expected.
[0061] Such undulation of the nominal stylus tip centre point 23
could be achieved for instance by varying the probe's stand-off
distance relative to the surface 17 of the workpiece 16 as it
traverses across the workpiece 16. Optionally, if the probe were
mounted on an articulated head, then this could be achieved by
varying the angular position of the probe about at least one of the
head's rotational axes.
[0062] In the embodiment described, the course of motion is
configured such that the nominal undulating motion 32 is configured
such that for a perfect workpiece 16 (i.e. in which the actual
workpiece corresponds exactly to the model workpiece) the probe's
14 stylus 22 is configured to oscillate between being over and
under deflected as it travels along the surface 17 of the workpiece
16, in between which the analogue probe 14 collects measurement
data within its preferred measurement range. For instance as shown
in FIG. 4 at nominal stylus tip position 24A, the stylus deflection
is such that the probe 14 obtains measurement data within is
preferred measurement range whereas nominal stylus tip positions
24B and 24C the stylus is respectively under and over deflected.
Accordingly, it can be seen from the embodiment of FIG. 4 that as
the stylus tip 24 travels across the surface only select portions
(illustrated by the dashed and dotted segments 34) where the stylus
deflection is within its preferred deflection range and hence only
select portions of the data obtained from the measurement probe
will be obtained within its preferred measurement range.
[0063] As will be understood, the amplitude A of undulating
position of the stylus centre tip is greatly exaggerated in FIG. 4
so as to aid illustration. As will be understood, the extent of the
amplitude A will vary depending on many factors including the
extent of the preferred measurement range, the extent the actual
physical range of a deflectable stylus, the nominal workpiece
dimensions and the expected level of variation in surface position.
Nevertheless, by way of example only, the amplitude A can be as
less than 5 mm, for example less than 2 mm, and for example greater
than 0.5 mm, for example 1 mm. Furthermore, the pitch P of the
undulating motion will vary depending on many factors, such as
those mentioned above and for example the density of measurements
required, and for example can be less than 100 mm and for example
greater than 10 mm, for example 50 mm.
[0064] Although the method of FIG. 4 results in only some of the
measurement data being obtained within the probe's preferred
measurement range, this method of measuring can help to ensure that
at least some measurement data is obtained within the preferred
measurement range despite the material condition of the workpiece
16 being different to what is expected. For instance, with
reference to FIG. 4, if the actual position of the workpiece 16 is
slightly offset such that its surface 17 is located a fraction
closer to the nominal probe tip centre line 32, then measurement
data will still be obtained, but rather than being obtained at the
points illustrated by dashed and dotted portions 34, the preferred
measurement data will be obtained at the peaks of the nominal probe
tip centre line 32.
[0065] FIGS. 5(a) and (b) illustrate an alternative embodiment in
which the part to be measured is unknown. In this case, no model of
the part is loaded into the PC at step 102, and step 104 comprises
generating a standard course of motion which can be used to obtain
measurement data about the unknown part. The part could be unknown
in that its shape and dimensions of at least one feature are
unknown and are to be determined. In the embodiment described, the
predetermined course of motion is configured such that the nominal
probe tip centre 23 moves back and forth along the same nominal
measurement line 19 on the surface 17 of the object 16, but at
different nominal distances from the surface 17, as illustrated by
dotted line 40. In the embodiment shown, each traverse across the
surface of the object is generally in a straight line, and also
constrained within one plane, however as will be understood this
need not necessarily be the case. Indeed, for example, each
traverse could involve causing the nominal stylus tip centre point
to undulate much like that shown in FIG. 4. Furthermore, the path
of each traverse could meander in a sideways direction, e.g. in a
side-to-side motion. Furthermore, the predetermined course of
motion need not necessarily move the nominal probe tip centre in a
back and forth manner. For example, each traverse could take place
in the same direction. Furthermore, each traverse could, for
example, comprise moving the nominal probe tip centre in a winding
(e.g. spiral) manner across the surface of the object.
[0066] As shown, the position of the analogue probe's preferred
measuring range will fall relative to the surface 17 of the object
16 over successive traverses. In particular its average position
along the nominal measurement line above the surface falls over
successive traverses. In the embodiment described, on the first
traverse, the stylus tip 24 does not deflect enough to enter its
preferred deflection range and hence no data is obtained within the
preferred measurement range. On the second traverse, the crest of
the surface 17 causes the stylus tip 24 to deflect within its
preferred deflection range and so measurement data is obtained
within the probe's preferred measurement range for a portion of the
pass, illustrated by the dashed and dotted portion 42. As can be
seen for the third and forth traverses, again the stylus is
deflected within its preferred measurement range so as to obtain
measurement data within the probe's preferred measurement range for
portions 42 of the passes. During step 110, these portions 42 of
measurement data that have been obtained within the preferred
measurement range can be filtered from the entire measurement data
set and collated so as to provide a new set of measurement data
regarding the object, all of which was obtained within the analogue
probe's 14 preferred measurement range. In the embodiment shown,
the nominal course of motion of the probe tip centre 23 is such
that the portions 42 of data obtained within the preferred
measurement range overlap between successive passes. However, this
need not necessarily be the case, which would therefore mean that
there could be gaps in any final data set that is created from data
obtained within the analogue probe's 14 preferred measurement
range. Furthermore, as with FIG. 4, the step toward the workpiece
between each pass is exaggerated to aid illustration. The actual
size of the step varies depending on a number of factors, including
the measuring range of the probe but typically could for instance
be as small as 0.2 mm and as large as 0.8 mm. Furthermore, although
it is shown that the nominal stylus tip centre point steps toward
the object after each traverse, this need not necessarily be the
case. For instance, the nominal stylus tip centre point could
nominally get progressively closer along the length of the pass
such that it gradually approaches the object along each
traverse.
[0067] In the embodiment described in connection with FIG. 5, the
analogue probe's stylus deflects beyond its preferred measurement
range, but never deflects beyond its maximum deflection threshold.
However, in other embodiments it might be that the shape and
dimensions of the object, and/or the predetermined path of relative
movement, is such that the analogue probe is at risk of its stylus
deflecting beyond its maximum deflection threshold. In this case,
during a scanning operation, the analogue probe's output can be
monitored to check for such a situation and take corrective action.
Such corrective action could be to halt and abort the scanning
operation. Optionally, such corrective action could be to adjust
the predetermined path of relative movement so that deflection of
the stylus beyond its maximum deflection threshold is avoided. For
instance, at the end of each traverse, it could be determined if on
the next, or a future, traverse the stylus is likely to deflect
beyond its maximum deflection threshold, and if so then adjust the
predetermined path of relative movement.
[0068] Such a rastering approach adopted described for unknown
parts in connection with FIG. 5 can also be useful even if the
nominal shape of the workpiece is known. For instance, with
reference to FIG. 6(a) there is shown a situation in which the
actual surface shape 17 of the workpiece 16 deviates from its
nominal surface shape 17' in that it has an unexpected dip 27.
Accordingly, if the stylus tip 24 were to follow a path 50
substantially parallel to the expected nominal surface shape 17,
then it would result in no measurement data being obtained within
the analogue probe's preferred measurement range for the dipped
part of the surface. However, as illustrated in FIG. 6(b) using a
path 52 which adopts the rastering approach enables the measurement
data obtained along the path 52 that was obtained within the
analogue probe's preferred measurement range (such data being
illustrated by dash and dot portions 54) to be filtered and
collated, so as to thereby provide measurement data that was
obtained within the analogue probe's preferred measurement range
for the entirety of the nominal measurement line 19 for the actual
surface shape 17.
[0069] As will be understood, the filtering could be achieved in
many different ways. For instance, it could be done at source, in
that only data obtained within the probe's preferred measurement
range is reported by the analogue probe and/or receiver/transmitter
interface 10. Optionally, all the data from the analogue probe is
reported, by only those measurements which were obtained within the
analogue probe's preferred measurement range are combined with
spindle (i.e. analogue probe) position data. In an alternative
embodiment, all analogue probe data is combined with spindle
position data, and then the combined data is subsequently filtered
to remove the combined data which contains analogue probe data
outside the preferred measurement range.
[0070] The above described embodiments filter for and collate data
that was obtained within the analogue probe's preferred measurement
range. As will be understood, this need not necessarily be the case
and instead for instance a method according to the invention could
filter for and collate data outside the preferred measurement
range, or indeed only report data that it outside the preferred
measurement range. This might be useful for instance when it is
only important to know when a part is out of tolerance (and
possibly for example by how much).
[0071] In the embodiments described above, the path along which the
analogue probe and object are moved relative to each other is
predetermined. In particular, the entire path is determined before
the scanning operation is begun. However, this need not necessarily
be the case. For instance, with respect to the embodiments
described in connection with FIGS. 5 and 6, the path of relative
movement could be generated on a traverse-by-traverse basis. For
example, a first traverse along the nominal measurement line could
be completed, and if it is determined that not all the measurement
data obtained along the nominal measurement line was within the
analogue probe's preferred measurement range, then a subsequent
traverse could be performed in which the position of the probe's
preferred measurement range is at a different position with respect
to the object along the traverse along the nominal measurement
line. This process could be continually repeated until measurement
data within the analogue probe's preferred measurement range has
been obtained along the entire length of the nominal measurement
line.
[0072] In other embodiments, the method of the invention can
comprise generating and executing (e.g. as part of a second
scanning operation) a new course of relative movement of the
analogue probe and object based on the measurement data obtained
during the previous scanning operation (e.g. during a scanning
operation according to the embodiments of FIGS. 4, 5 and 6). The
new course of relative movement can comprise the analogue probe
traversing substantially the same line of measurement across the
surface of the object. However, in this case the relative movement
can be controlled such that the relative position of the analogue
probe and object is such that the analogue probe obtains
measurements within its preferred measurement range for a greater
proportion of the measurement path than for the previous
measurement of the object. In particular, the new path of relative
movement for the analogue probe and object to follow can be
configured such that the analogue probe obtains measurement data
within its preferred measurement range along substantially the
entire length of the same nominal line.
[0073] This is the case shown in FIG. 7, which is a replication of
FIG. 6(b), except that solid line 56 illustrates the path of the
probe tip centre which was generated from the data obtained during
the scan illustrated by the dotted line 52.
[0074] The above described scanning operations can be performed at
high speed (e.g. with the workpiece sensing part (e.g. the stylus
tip 24) and object travelling relative to each other at least at 16
mm/s, preferably at least at 25 mm/s, more preferably at least at
50 mm/s, especially preferably at least at 100 mm/s, for example at
least at 250 mm/s) because it doesn't matter whether the probe 14
obtains data outside its preferred measuring range.
* * * * *